This invention relates to direction indicating apparatus and, more particularly, to such apparatus for providing an indication of the commanded direction of rotation of a rotary member just as, or momentarily prior to the instant that the rotary member commences rotation in the commanded direction.
In many motor control systems, it is both desirable and necessary to provide an indication of the actual direction of rotation of the motor. This, of course, corresponds to the actual direction of movement of a driven member. One example of apparatus which incorporates such a motor control system is tape recording and/or playback apparatus, such as an audio tape recorder, a video tape recorder (VTR) or a digital data recorder. In recorders of this type, information is recorded on and/or reproduced from a driven magnetic tape. Usually, the tape is driven by a combination of capstan and pinch roller, with the capstan being rotated by a controlled motor.
In apparatus of the type described above, a capstan-drive motor is selectively energized in accordance with the actual direction of rotation thereof. For example, if the motor is driven in a forward direction under the control of a typical motor control circuit, and if the direction of rotation then is to be reversed, such as for rewinding or reviewing a portion of the driven tape, the motor cannot be reversed instantaneously. Rather, the motor first must be decelerated and then, when motor speed is approximately zero, reversely energized for rewinding the tape. In such an operation, it is important to know the actual direction of rotation of the motor for the purpose of properly supplying forward and reverse energizing signals thereto. If a reverse energizing signal is supplied to the motor when the motor actually is rotating in a forward direction, possible damage thereto may occur.
Because of inertia inherent both in the motor, the capstan and the various tape drive elements, it is known that a substantial time delay is present between a change in the commanded direction of motor rotation, such as a change in a direction command signal produced by a suitable control circuit, and a corresponding change in the actual direction in which the motor rotates. This delay may vary as a function of mechanical constants, energizing and de-energizing, or braking, signals, motor currents, instantaneous motor speed, and the like. Hence, such delay is unpredictable and is not easily accounted for in selectively energizing, de-energizing or reversely energizing the motor. Consequently, there is a need for providing an indication of the actual direction of rotation of the motor, especially when the motor is controlled during a transition period whereby it is reversely driven.
It has been proposed heretofore to provide two phase-displaced sensors for sensing the rotation of a motor, these phase-displaced sensors being spaced apart by 90.degree. such that the respective signals produced thereby provide indications both of speed and direction of rotation of the motor. For example, if a magnetic-type disc is secured to the motor shaft so as to be driven thereby, a pair of 90.degree. phase-displaced magnetic sensors will produce respective pulse signals whose frequency corresponds to the rotary speed of the motor and whose phase relation represents the direction of rotation of the motor. That is, if the pulses generated by the sensors differ from each other by 90.degree., then it is determined that the motor is rotating in one direction. If the phase difference between these pulses is 270.degree., then it is determined that the motor is rotating in the opposite direction. As another example, a shutter disc provided with a series of circumferential apertures may be secured to the motor shaft and rotated past a pair of 90.degree. phase-displaced photosensors, such as phototransistors. If light is projected onto the photosensors, this light will be interrupted as the shutter disc rotates, thereby resulting in two trains of pulses that are phase displaced with respect to each other by an amount corresponding to the direction of rotation of the motor, and the frequency of the pulses representing the speed of rotation of the motor.
While the use of a pair of sensors generally is adequate to provide an indication of the direction of rotation of the motor, this generally adds to the cost of manufacture and assembly of motor control apparatus. Furthermore, some manufacturers of motors are providing same with a single sensor integrally constructed therewith for providing a signal representing the actual speed of rotation of the motor. Such a motor would have to be dismantled and then reconstructed if a second sensor, heretofore needed for providing an indication of direction of rotation, is to be added. From the foregoing, it is appreciated that there has been a need for utilizing only a single sensor which provides merely an indication of the speed, but not direction, of rotation of a motor, to obtain an indication of rotational direction as well.